Motorized three-wheeled vehicles are well known in the art. Such vehicles are typically off road type or all terrain vehicles (also known as “ATVs”). Two different configurations of three-wheeled vehicles are generally known. The first configuration has two wheels at the front and one wheel at the back of the vehicle. The second configuration has one wheel at the front and two wheels at the back.
Regardless of the particular configuration for a three-wheeled vehicle, those skilled in the art recognize that three-wheeled vehicles are intrinsically less stable than four-wheeled vehicles, such as automobiles. Several factors contribute to this instability. One of them concerns the fact that a three-wheeled vehicle has only three contact points with the ground instead of four contact points.
It should be noted at the outset that the intrinsic instability of a three-wheeled vehicle versus a four-wheeled vehicle should not be understood to mean that a three-wheeled vehicle is unstable to the point that it is dangerous to a user. To the contrary, as would be understood by those skilled in the art, some designs for three-wheeled vehicles are inherently more stable than certain four-wheeled vehicles.
Another factor that affects the stability of a vehicle is the center of gravity of the vehicle. The height of the center of gravity of a vehicle is measured as a distance from the ground when the vehicle is at rest. The center of gravity changes based on the rider position and the type of seating arrangement provided.
A straddle seat type vehicle positions the rider higher from the ground and, as a result, typically creates a vehicle with a higher center of gravity than a vehicle that has a recumbent type seat, which is more stable but requires additional space and offers less rider control. Recumbent type seats include bucket seats, etc. of the type usually found in four-wheeled vehicles. Recumbent seat configurations generally position two riders side by side.
While straddle seats may alter disadvantageously the center of gravity of a vehicle, they offer certain advantages that are not available with recumbent seats. In particular, straddle seats allow a more compact riding position and permit the rider to lean into a turn for enhanced handling. Straddle seats also may provide a second passenger seat behind the driver seat, if desired, but the additional rider also raises the center of gravity of the vehicle.
An advantage of a tandem vehicle (riders positioned behind one another) is that the center of gravity of the vehicle remains symmetrically positioned if there are one or two riders. In contrast, on a light-weight, side-by-side, recumbent three-wheeler, when only the driver is present, the center of gravity is not located in the same position as when there are two riders in the vehicle. When only a driver is present in a three-wheeled vehicle with side-by-side, recumbent seats, the center of gravity will be offset from the longitudinal centerline of the vehicle in a direction toward the driver. As would be appreciated by those skilled in the art, this offset may have an affect on the handling performance of the recumbent-seated vehicle.
Other factors that affect stability include the distance between the wheels. On a vehicle, the wheel base refers to the distance between the front axle and the rear axle. The front track, on the other hand, refers to the distance between the center of the two front wheels, which is typically very similar to the distance between the center of the rear wheels in a four wheeled vehicle. A larger distance between the wheels (whether it be the wheel base or the wheel track) enhances the stability of the vehicle, but creates a larger vehicle, in terms of over all length and width, that may be less maneuverable because of the vehicle's increased size.
When operating any vehicle, especially a three-wheeled vehicle, stability is a concern during turning. When negotiating a curve, a vehicle is subject to centrifugal forces, as is readily understood by those of ordinary skill in the art of vehicle design. Generally, a higher center of gravity causes the vehicle to be more sensitive to centrifugal forces than a vehicle with a lower center of gravity.
Three contact points representing the tires define a triangle and both lateral sides of the triangle are closer to the vehicle CG than on a four-wheeled vehicle. A four-wheeled vehicle defines a square, the vehicle CG is therefore at a longer distance from the lateral side of the square. The dynamic displacement of the CG when the vehicle is cornering may pass over the lateral lines therefore getting over the vehicle stability threshold.
Three-wheeled vehicles raise special stability concerns since there is a smaller total tire contact area (with the ground) as compared with four-wheeled vehicles. Usually three-wheeled vehicles have a smaller mass. Therefore, the contact pressure of the tire on the ground is reduced. Moreover, if a straddle seat is employed, the center of gravity can be relatively high, as compared with that of a recumbent three-wheeled vehicle.
To equip a three-wheeled vehicle for road use, road tires must be employed. In a poorly designed vehicle, at high speeds or in sharp turns, the centrifugal forces generated on a road could exceed the traction threshold of a road tire, which could cause one or more of the tires to slip on the road surface. The slippage may be so severe that the vehicle could oversteer or understeer under certain circumstances.
Tire slippage is a phenomenon that is not unique to three-wheeled vehicles. The car industry experienced a similar stability concern with vehicles that have a high center of gravity. Sport Utility Vehicles (SUVs) tend to have high centers of gravity and are particularly sensitive to weight transfer and may tend to roll over when negotiating high lateral acceleration curves. As a result, electronic stability systems (ESS) have been developed to improve the stability of such vehicles.
As would be appreciated by those skilled in the art, modem road tires can offer considerable grip on a road surface. The gripping force of modern road tires can be so strong, in fact, that a vehicle with a high center of gravity vehicle may be subjected to forces that may cause the vehicle to exceed its rollover threshold. If the rollover threshold is exceeded, one or more of the vehicle's wheels on the inner side of the curve may lift off of the road surface. Under such circumstances, if the rider continues to apply a lateral acceleration to the vehicle, the rider may be able to roll the vehicle over. Rollover can also be experienced under severe oversteering conditions if the tires suddenly recover traction with the ground.
Electronic stability systems (ESS) are designed to electronically manage different systems on an automotive vehicle to influence and control the vehicle's behavior. An ESS can manage a considerable number of parameters at the same time. This provides an advantage over an automotive vehicle merely operated by a person since the driver can only manage a limited number of parameters at the same time. A typical ESS takes several inputs from the vehicle and applies different outputs to the vehicle to influence the vehicle's behavior. Examples of inputs include steering column rotation, the longitudinal and transverse acceleration of the vehicle, the engine output, the detection of the presence (or absence) of a rider and a passenger, the speed of the four wheels and the brake pressure in the wheel's brakes. Traditional ESS's use inputs from all four wheels. Some low-cost systems use reduced wheel speed inputs, but this does not result in optimal behavior. Inputs from suspension displacement and brake and accelerator pedal displacement can also be provided to the ESS.
The outputs from the ESS affect the automobile's behavior by generally independently managing the brakes on each wheel, the suspension, and the power output of the engine in order to improve the automobile's handling under certain circumstances. Since ESS's have been specifically developed for four-wheeled vehicles and rely on inputs provided by a four-wheeled vehicle, it is not convenient to adapt this kind of system to a three-wheeled vehicle. This is especially true since an ESS typically uses inputs from each of the four wheels independently and uses the braking system independently on all of the wheels. It is also possible to adapt suspension settings corresponding to the four wheels to change the behavior of the vehicle.
As would be appreciated by those skilled in the art, there are many ways in which suspension behaviors can be modified. For example, the internal valve setting(s) in one or more of the shock absorbers may be changed mechanically or electronically. Alternatively, the spring pre-load may be adjusted. Additionally, the fluid viscosity in the shock absorber may be adjusted by subjecting a magnetorheological fluid to an external electric or magnetic field.
A three-wheeled vehicle configured with a single wheel at the rear of the vehicle does not provide all the information/data input required by a four-wheeled vehicle ESS. For example, there is only one rear wheel from which the ESS can receive input on speed. Moreover, on a vehicle having two rear wheels, when the brake is applied to one wheel, a “yaw moment” is generated about a vertical axis passing through the center of gravity of the vehicle. On a vehicle having only one rear wheel, the rear wheel is positioned in the same plane as the longitudinal axis of the vehicle, which makes it difficult to generate any “yaw moment” by applying the brake to the rear wheel. However, it is known that a very wide single rear tire can generate a small “yaw moment” under strong lateral acceleration due to lateral displacement of the tire contact patch. A vehicle experiencing understeer has limited cornering ability on the understeering axle. In order to create a stabilizing yaw moment, a single brake force must be applied to an inner rear wheel, since this will create a restoring moment by capitalizing on the surplus cornering force available from that tire. It is understandable that this may cause a problem when there is only one centered rear wheel.
A system that improves stability of a three-wheeled vehicle is desired in the industry. There is especially a need for such a system that can operate based on the detection of roll of the vehicle, without the assistance of an electronic control unit (ECU).